LTE (Long Term Evolution), specifications of which have been designed in 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, supports Frequency Division Duplex (FDD) in which communication is performed by using a downlink frequency and an uplink frequency.
In such a mobile communication system, a user terminal feeds back, to a base station, channel state information (CSI) corresponding to a channel state in the downlink frequency on the basis of a downlink reference signal that is transmitted from the base station by using the downlink frequency (for example, see Non Patent Literature 1).
The base station performs downlink transmission control on the basis of the CSI fed back from the user terminal. The downlink transmission control is downlink multi-antenna transmission control, for example.
In order to realize sophistication of the downlink transmission control as described above, more highly accurate CSI is needed.
However, there is a problem in that in the current specifications, the number of bits in channel state information is fixedly set, and thus, it is not possible to feed back more highly accurate CSI.